Method of stabilization of thermionic sources and thermionic source obtained by application of said method or a like method



United States Patent ()fiice 3,423,583 Patented Jan. 21, 1969 4 Claims ABSTRACT OF THE DISCLOSURE The present invention is concerned with stabilized thermionic sources and their method of preparation. The thermionic sources according to the present invention are designed for use in mass spectrometry. The sources are prepared by coating a metallic heating filament with a layer of refractory substance possessing poor thermal conductivity. The coating preferably is carried out by depositing on the surface of the filament a layer of the refractory substance and calcining the structure thus obtained so as to adhere said layer to the filament. The resulting stabilized filament is then coated with a layer of a mixture comprising the product to be analyzed and the refractory substance. The filament is again calcined so as to produce an outer layer composed of the latter mixture. Titanium pyrophosphate, magnesium aluminate and nickel oxide have been found to be the preferable refractory substances.

When a solid thermionic source is employed for the purpose of performing an analysis by mass spectrometry, there always takes a rapid decrease in the ion current as a result of the evaporation of the sample being studied. This decrease makes it more difficult to interpret the diagrams than in the case of utilization of a gas source in which such a phenomenon is not perceptible. For this reason, it has appeared advantageous to reduce this process of ion current decrease by means of an efiect which would prevent evaporation by overheating of the sample to be analyzed.

To this end, the invention has for its object a method of stabilization of a thermionic source which can be utilized in mass spectrometry and which makes it possible to obtain a constant thermionic emission.

The method in accordance with the invention consists in depositing on a metallic filament which serves as a heating source a layer of substance which is capable of providing as a result of heating a uniform layer having a smooth surface, in effecting the slow calcination of the filament which is coated with said substance until there is formed a layer which effectively adheres to the filament, in charging the aggregate thus obtained with a mixture consisting of said substance and of the product to be analyzed by mass spectrometry and in calcining the aggregate.

The thermionic source as thus prepared can be employed for the purpose of performing an analysis by mass spectrometry; said source consists of a heating filament which is coated with a layer of stabilizing substance which is in turn coated with a layer of a mixture of the stabilizing substance and of the substance to be analyzed.

The stabilizer must be refractory and have poor thermal conductivity in order to withstand without melting the high temperatures to which the filament is brought at the time of heating. The stabilizer must be such that, after it has been applied on the filament and after said filament has been heated, said stabilizer is present in a uniform layer having a smooth surface. The stabilizing substance thus prepared is essentially intended for temperature stabilizatron.

Among the products which meet the foregoing conditions, there can be mentioned by way of example the following substances which can be employed either separately or in combination:

Titanium pyrophosphate (TiP O- Magnesium aluminate (MgAl O Nickel oxide (NiO).

A large number of other metal salts are also suitable for the practical application of the invention. It is merely necessary to ensure that such salts meet the following conditions: they must be refractory, they must have poor thermal conductivity and they must provide, once they have been applied on the filament and heated, a uniform layer having a smooth surface. There can be mentioned by way of example the salts of calcium, strontium, barium, zirconium, thorium and hafnium.

One non-limitative example of application of the method in accordance with the invention to the preparation of a stabilized thermionic source will now be described. The metallic heating filament consists of a metal which has a high melting point and which can advantageously be platinum, rhenium or tungsten.

The stabilizer is prepared in the form of gel and is deposited on the filament by means of a platinum spatula; the aggregate is then subjected to a first calcining treatment by sending an electric current through the filament, thus increasing its temperature by Joule effect. The heating must be performed with care in such a manner as to ensure that the layer of stabilizing substance effectively adheres to the filament after calcination.

The filament which is thus covered with stabilizer is then charged with a layer of stabilizing substance which is mixed with the product to be analyzed.

The quantities employed must be small since it would prove undesirable to overcharge the filament. The calcination is then performed under the same conditions as before. When these different operations have been completed, the filament appears in the form of a thin layer of stabilizer which adheres to the metal in a perfect manner and the thickness of which can vary between 0.2 and 1 mm. but is preferably equal to 0.5 mm. Under the microscope, the surface of said layer is of uniform appearance although not glass-like.

One preferred method of operation of the source in accordance with the invention will now be described in the case in which the said source as thus prepared is employed for the purpose of performing an isotopic analysis by mass spectrometry. The filament which is prepared in the manner which has been described above is fitted in the source of the spectrometer, then heated very progressively until the vacuum of the source is restored. Rapid heating is harmful since it is liable to result in separation of the activating layer which has been deposited on the filament.

As soon as the vacuum of the source has been brought to a correct value, the appearance of the mass spectrum is studied and there are first of all observed the impurities which are contained in the stabilizer and which can be rapidly caused to disappear by a slight increase in temperature. The masses corresponding to the elements of the product to be analyzed then appear and the emission can be very easily regulated.

The source then remains stable throughout the analysis (this latter can last several hours) without any appreciable decrease in.the ion current.

A thermionic source in accordance with the invention has the great advantage, in particular when performing an analysis by mass spectrometry, in that the product to be analyzed can be utilized in solid form, thus requiring only a minimum number of manipulations. Moreover, a source of this kind makes it possible to perform isotopic analyses of long duration without any fluctuations which are higher than those produced by a gas source.

What we claim is:

1. A method for producing a stabilized thermionic source designed for use in mass spectrometry which method comprises the steps of:

(a) depositing on a metallic heating filament a layer of a refractory substance of poor thermal conductivity which substance provides upon heating a uniform coating having a smooth surface;

(b) calcining the filament coated with said substance to thereby form and adhere said uniform coating to said filament;

(c) charging the thus obtained coated filament with a mixture composed of said refractory substance and the product to be analyzed by mass spectrometry; and

(d) calcining the charged coated filament.

2. A method according to claim 1 wherein said substance is selected from the group consisting of titanium pyrophosphate, magnesium aluminate and nickel oxide.

3. A thermionic source designed for use in mass spectrometery which comprises a heating filament composed of a conductive substance having a high melting point coated with a uniform layer of a refractory substance having poor thermal conductivity and additionally coated with a layer comprising a mixture of the product to be analyzed by mass spectrometry and said refractory substance.

4. A thermionic source according to claim 3 wherein said refractory substance is selected from the group consisting of titanium pyrophosphate, magnesium aluminate and nickel oxide.

References Cited UNITED STATES PATENTS 2,710,354 6/1955 Inghram et a1. 250-41.9 2,756,341 7/1956 White 250-419 3,195,004 7/1965 Hassett 313345 X WILLIAM F. LINDQUIST, Primary Examiner. 

